The noted applications require simple, cheap and rubbed spectrophotometers which are easy to maintain, can be purchased in large quantities and can be used in harsh environments. In addition, accuracy requirements such as photometric stability, wavelength resolution, and spectral reproducibility must be equal to or better than that possessed by state-of-the-art instruments. The objective of the present invention is to achieve the above requirements.
Photometer devices are known in the prior art that use a single light source, a single detector and a dual beam fiber optics that splits the light into a reference beam and a sample beam. Such devices are shown, for example, in U.S. Pat. No. 4,061,428 and an article by Konnerth et al entitled "In-Situ Measurement", IEEE Transactions (7/75). A common feature of such prior devices is the use of a chopper wheel having apertured portions to allow both the reference beam and the sample beam to pass through a selected filter in the aperture. However, considering the device of the '428 patent, it is noteworthy that the triggering apertures 33 in chopper wheel 22 are identical in number to the filter apertures 24. That is, there are disclosed 3N apertures 24 and 3N apertures 33. Further, the angular placement of apertures 24 is the same as such placement of the bifurcated fiber optics 17 and 14. As a result, the only way to prevent both the sample beam and the reference beam to arrive at the detector simultaneously (an unacceptable result), is to position an oscillating shutter 46 adjacent to chopper 22. (Although there are N more apertures 38 also in chopper 22, these are used to detect red versus green versus blue filters, and do not control whether a reference beam or a sample beam is in place). Such a use of a chopper and a shutter has a decided disadvantage--there are at least two moving parts required. Moving parts are well-known to be the cause of mechanical breakdown.
Hence, prior to this invention, the problem has been that a spectrophotometer has not been provided with a single light source, a single detector, split optics for dually detecting both a sample and a reference using the single source and single detector, and means for triggering and collecting either the sample beam or the reference beam for detection, wherein only one moving part is present in the optics.
In the Konnerth article noted above, there appears to be only one moving part in the spectrophotometer of FIG. 1. However, the article does not describe the use of a triggering mechanism whereby the motor and chopper inform the computer where the chopper is. Instead, the computer drives the motor. This is insufficient because the accuracy and timing of the positioning has to be exact. For such an arrangement, the costs are high. A more preferred system is one in which the positioning can have wide tolerances because the actual rather than theoretical positioning is detected.
More recent attempts to provide spectrophotometers have not solved the noted problems. U.S. Pat. No. 4,648,714 teaches a dual beam spectrophotometer having a single moving part (chopper 58 with four filter apertures 17(a)-17(d)). However, it does this at the expense of two detectors (19 and 37). The use of two different detectors is well known to be a source of drift error.